Electromagnetic device



June 17, 1958 0, RICE 2,839,631

ELECTROMAGNETIC DEVICE Filed July 28, 1954 Inventor:

( borneg.

ELECTRtJMAGNi-ETIC DEVICE Neil 8. Rice, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 23, 1954, Serial No. 446,325

9 Claims. (Cl. ZOO-87) My invention relates to electromagnetic devices, and more particularly to a hinged armature type of electromagnetic device adaptable for use as an alternating current relay, contactor and the like.

In an electromagnetic relay energized by alternating current it is common practice to use a slotted pole face and a shading coil. Main and out-of-phase components of total flux are thereby established, and the armature of the relay will experience some magnetic pull at all times. As a result, the electromagnetic relay will operate without excessive chattering which otherwise would produce objectionable noise and wear. Frequently the unshaded portion of the pole face is recessed to provide an air gap in the path of the main component of flux with the armature in its attracted, picked-up position. In this manner, the reluctance to the main flux is increased and the magnitudes of the main and out-of-phase components of flux are balanced, thereby reducing the maximum and raising the minimum values of net force exerted on the armature by the combined fluxes during each cycle of alternating current. Maximum utility of the flux components is thus obtained, and it is possible to employ a relatively more powerful bias element to return the armature to its unattracted, reset position with maximum dispatch upon deenergization of the electromagnet.

It is well known that as the armature picks up, it closes or shortens an air gap in the magnetic circuit of the relay thereby reducing the reluctance to the flux. Therefore, the magnitude of energizing ampere turns required to produce the necessary flux to pick up the armature from its reset position may be much greater than the magnitude required to retain the armature picked-up. Conversely and more specifically, for the electromagnet having balanced main and out-of-phase flux components the maximum value of ampere turns to release the armature for resetting may be as low as 55% of the value required to pick up the armature. This characteristic is undesirable for many electromagnet applications, particularly Where the value of current required for resettin is less than the normal value of current for which no pick up is desired. In this circumstance, an electro-magnetic relay might fail to reset if the controlling condition returns only to its normal value and does not deenergize the electromagnet after an operation. It is possible to increase the ratio of reset value to pick up value by suitable adjustment of the initial position of the armature, but this method is limited by the necessity of maintaining certain minimum clearances and by the physical limitations of the particular electromagnetic device.

Accordingly, it is an object of this invention to provide an improved electromagnetic device of the hinged armature type having a relatively high ratio of reset value of ampere turns to pick up value of ampere turns while maintaining quiet operation without objectionable chattering.

It is a further object of this invention to provide an improved electromagnetic device of the hinged armature ited States Patent 0 type having a means to adjust readily the reset value thereof.

A still further object of this invention is the provision for an electromagnet having a hinged armature of means for adjusting the reluctance of the flux path between the pole face and the armature.

Another object of this invention is to provide an electromagnetic device including an armature normally biased to a reset position with a means to partially disable the bias element while the armature is reset.

A more specific object of this invention is the provision, for an electromagnetic relay including a slotted pole face and a shading coil which divide the total flux into two out-of-phase components, of means for adjusting the relative magnitudes of and the phase angle between the two components by relatively adjusting the length of air gap between each portion of the pole face and the armature.

In carrying out my invention in one form, I utilize an alternating current electromagnetic relay provided with a slotted pole face having a shading coil, thereby to produce main and out-of-phase components of flux, and having a hinged armature spring biased to its unattracted, reset position. i provide the armature with a resilient element that engages a stationary stop whenever the armature is in its reset position to produce a force gradient opposing and partially disabling the bias spring. Thus the magnitude of magnetic pull required to attract or pick up the armature from its reset position is less than otherwise would be required. I locate the hinge of the armature outside the plane of the surface of the pole face so that the armature when picked-up will engage only one edge of the pole face and define an oblique angle therewith. The pole face is made rotatable to permit adjustment of the length of air gap through which each component of flux must pass between the pole face and the inclined armature. This adjustment will affect the reluctance to flux and consequently will vary the relative magnitudes of each component of flux and the phase angle therebetween. It is possible, therefore, to select a radial position of the pole face in which an unbalance between the main and out-of-phase components of flux will result in release of the armature for resetting whenever the magnetic pull exerted by the electromagnet is reduced only a slight amount below the value required for pick-up, while operation of the relay will be maintained with no excessive chattering.

My invention will be better understood and further objects and advantages will be apparent from the following description taken in conjunction with the accompanying drawing in which Fig. l is a front elevational view of an electromagnetic relay embodying my invention; Fig. 2

is a sectionalized side view of the relay taken substantially on line 2--2 of Fig. l and showing the armature in its unattracted position; Fig. 3 is a side view of the lower portion of the relay, partially in section, showing the armature in its attracted position; and Fig. 4 is a plan view of the armature with the pole faces and shading coil superimposed thereon.

Referring now to Fig. 2, the illustrated embodiment of my invention comprises an electromagnct including a U- shaped frame n'rember 11, an energizing winding 12 wound on an insulating spool 13 and an axial disposed core member 14. To control the energization of wind ing 12, connections, not shown, are made from the Winding to a remote source of alternating current.

The core member 14 comprises a cylindrical sleeve 15 having a cooperating pole head 16 and inner sleeve 17 and an adjustable core element 18. A shank end of the inner sleeve 17 is threaded into one end of sleeve 15 and is locked therein by binding engagement between the two sleeves at cooperating shoulders 19 of their respective threaded portions. A flange 21) having a hexagonal periphery is formed at the other end of inner sleeve 17.

An aperture in the closed'end portion of the U-shaped frame member 11 freely admits the shank of the inner sleeve 17 but will not pass the larger cylindrical sleeve or flange 2t). Sleeve 15 is disposed in parallel relation between the legs of frame 11, and sleeve 1'7 is locked in place to capture frame member 11 between the sleeve 15 and the flange 20. A lock washer 21 is employed between the flange and frame member 11 to supply a clamping force thereby to hold the core member 14 firmly connected to frame 11. However, by using a wrench to apply torque to flange 20, the core member 14 can be rotated about its axis to adjust its radial position.

The pole head 16 is connected to the other end of cylindrical sleeve 15, as shown in Fig. 2. An internal end extends into sleeve 15 and forms a pressure fit therewith. The other end of pole head 16 extends externally and terminates in a disk-like portion having a substantially fiat outer surface generally defining a plane that is perpendicular to the axis of the core member 14. The disk like portion of pole head 16 overlaps one flange of the insulating spool 13 and cooperates with the closed end portion of the frame 11 to define the position of spool 13 together with winding 12. A slot 24 diametrically divides the disk-like portion of pole head 16 into two projecting pole faces 22 and 23. To reduce the surface area of pole face 22, peripheral section of the disk-like portion of pole head 16 is cut away along a line parallel to the slot 24, as can be seen in Fig. 4. Pole face 23 is semi-circular and is encircled by a D- shaped shading coil 25. The shading coil 25, as shown in the illustrated embodiment of my invention, comprises a unitary magnetic member having a straight portion disposed in slot 24 between pole faces 22 and 23 to form a pressed fit therewith. This coil acts in a well-known manner to delay the flux passing through it and thereby to establish in pole face 23 a component of flux which is out of phase with the flux of unshaded pole face 22.

The core element 18 is a solid, cylindrical member threaded into the inner sleeve 17 and extending through sleeve 15 as far as pole head 16, as shown in Fig. 2.

A hexagonal head 26 is formed at the external end of core element 18 to receive a wrench whereby the element 18 may be rotated. In this manner the distance that the core element 18 extends into sleeve 15 can be adjusted to vary the length of air gap between the element 18 and pole head 16. A lock nut 27 is employed to securely bind the core element after desired adjustment has been made. All parts of the core member 14 consist of magnetizable material, and the core member provides one leg in the magnetic circuit of the electromagnet. Adjustment of the core element 18 will vary the reluctance to flux of this leg. The greater the air gap between core element 18 and pole head 16, the greater will be the reluctance and the less will be the flux produced by the electromagnet for a given value of ampere turns.

The frame member 11 is constructed of magnetizable material and provides a path for the flux between core member 14 and an armature member 28. As shown in side view Fig. 2, the right hand leg of the U-shaped frame member 11 pivotally supports the armature 28. For the sake of clarity throughout this specification, I designate the right hand side of Fig. 2 as the rear of the electromagnetic relay and the left hand side as the front. Therefore, the right hand leg of frame 11 will be referred to hereafter as the rear leg, and the left hand leg hereafter will be called the front leg. The rear leg of frame 11 is notched laterally at 29 near its open end to receive a cooperating notched portion of armature 28. The armature extends forward from the rear leg across the pole head 16 and is movable between an attracted and an unattracted position in a plane 4 substantially perpendicular to the surfaces of the pole faces.

The armature member 28 includes a thin, fiat magnetizable plate 30 disposed adjacent to the pole head 16 connected to an insulating member 31 by a pair of rivets 32. Opposite sides of plate 31) are notched at 33 and interfitted with the laterally notched portion 29 of the rear leg to form a pivotal connection. This pivotal connection is disposed beyond the plane of the pole faces whereby plate 31), whenever the armature 28 is in its attracted position, will engage the pole head 16 at one point only and will form an oblique angle with the surfaces of the coplanar pole faces 22 and 23. In the illustrated embodiment of my invention, the plate 30 will engage only the front edge of the pole head, as shown in Fig. 3. An air gap of tapering length is defined by plate 30 and the pole faces, and this gap provides air paths for the fluxes in the pole faces. The air paths together with magnetizable plate 30 complete the magnetic circuit of the electromagnet.

The armature member 28 carries a resilient switch member 34, a leaf spring element 35 and a rigid guard member 36 each mounted in cantilever relation on insulating member 31 by a common rivet 37 located near the rear end of the armature, as can be seen in Figs. 2 and 3. These members extend forward from rivet 37 and are disposed in a vertical relationship, as viewed in the drawing, with resilient switch member 34 immediately below a forward projecting portion of insulating member 31, and with leaf spring 35 in spaced apart alignment below switch member 34. Guard member 36 engages the lower surface of the forward end of leaf spring 35 to deflect this spring to a prestressed position. Switch member 34 carries at its forward end a transversely disposed current conducting contact arm 38 Contact arm 38 is shaped substantially in the form of a W with laterally flanged ends each supporting a movable switch contact 39. The switch member 34 is connected to the central vertex of the W. Pivotal movement of armature 28 carries contact arm 38 between two preselected positions.

To define the two preselected positions of contact arm 38, a bracket 41 of insulating material is mounted by a pair of screws 41 to the front leg of the U-shaped frame member 11. This bracket supports a target 42, a pair of fixed switch contacts 43 and a fixed stop 44. The contacts 43 and stop 44 are disposed on opposite sides of the contact arm 33 and define the preselected positions thereof, as can be seen in Figs. 2 and 3.

Each of the fixed switch contacts 43 is mounted on bracket 40 by a pair of screws 45. A flexible connector 46, from a remote electrical circuit to be controlled by the relay, terminates at each fixed switch contact 43 and is connected thereto by means of solder or the like. The pair of fixed switch contacts 43 are disposed in cooperating relationship with the pair of movable switch contacts 39. The contacts 43 and 39 are in engagement whenever the armature 28 is in its attracted position, thereby completing an electrical circuit from one connector 46 to the other through contact arm 38.

The fixed stop 44 is formed by an integral laterally disposed lower lip of bracket 40 facing toward the rear of the relay. Whenever the armature 28 is in its unattracted position, the lower extremities of the W-shaped contact arm 38 engage stop 44 to determine the position of the contact arm 38. Similarly, while the armature 28 is in its unattracted position, leaf spring element 35 is in engagement with stop 44 at a point approximately midway between the lower extremities of the contact arm 38.

Armature Z8 is biased to its unattracted position by a tension spring 47. This spring is connected between a rearward projection of the closed end of frame member lland the rear end of plate 30 which extends behind 13 its pivotal connection at 29. As can be seen in Fig. 2, the force exerted by the bias spring 47 pulls the rear end of plate 30 upward which urges the forward portion of armature 28 counterclockwise about pivot 29 away from the pole faces 22 and 23.

Movement of the armature 23 to its attracted position operates target 42. Target 42 is a flat rectangular shaped plate of insulating material slidably mounted immediately behind three windows 48 located in bracket 40, as can be seen in Figs. 1 and 2. The forward surface of target 42 is colored black with the exception of three spaced apart horizontally disposed sections, 49, which are preferably colored orange. The three orange sections 49 will be exposed through windows 48 whenever target 42 is in its lower position. A resilient latch strip 50 is mounted on frame member 11 by a screw 51 and releasably engages the lower end of target 42 to hold target 42in a raised position. A downward projection of latch 50 extends into cooperating relationship with the forward projecting portion of insulating member 31, and upon armature 28 reaching its attracted position, latch 50 will be tilted away from target 42, as shown in Fig. 3, thereby releasing the target which will slide under the force of gravity to its lower position wherein the orange sections 49 will be exposed.

To return target 42 to its raised position after armature 28 has returned to its unattracted position, a reset lever 52 is provided. Lever 52 is L-shaped with a laterally disposed leg 53 pivotally connected to frame 11 and a vertically disposed leg extending along one side of the relay suitable for manual operation. Movement of reset lever 52 operates a reset element 54 which is pivotally supported on frame 11. This element, which includes a pair of arms disposed to form a V, is held by a yieldable means in a normal position as shown in Fig. 2. A projection 55 of the lateral leg 53 of lever 52 cooperates with one arm of element 54, and counterclockwise movement of the lever, as viewed in Fig. 2, will cause element 54 to move clockwise about its pivot. This movement will lift target 42 as the other arm of element 54 releasably engages, and lifts the lower edge of a protrusion 56 extending from the rear surface of target 52. In the raised position of target 42, resilient latch will engage the lower end of the target. Upon release of reset lever 52, element 54 will be urged by the yieldable means to pivot counterclockwise to its normal position.

From the foregoing detailed description of the structure of my electromagnetic relay, its mode of operation may now be readily followed. Five separate principal forces interact to control the movement of the armature 28.

Tension spring 47 applies a force that biases armature 28 to its unattracted or reset position. Due to a constant spring characteristic, this force will vary in a linear relation to the amount of spring deflection. Thus, as the armature 28 travels toward its attracted or picked-up position, spring 47 will be extended and the bias force will increase.

It may be assumed that the relay is physically oriented as shown in the drawing. Therefore, gravity will act to apply a constant force urging the armature toward its lowest position which corresponds to its reset position.

Energization of the electromagnet winding 12 will produce a magnetic field tending to pull armature 23 toward its picked-up position. The force exerted on the armature 28 by the electromagnet is proportional to the square of the magnetic flux in the air gap between the pole faces 22 and 23 and the plate 30 of armature 28. The quantity of flux in the air gap is a direct function of the magnitude of ampere turns of winding 12 and an inverse function of the reluctance of the air gap. The reluctance will vary as a direct function of the length of air gap. Therefore, as the armature 28 moves toward the pole faces 22 and 23, assuming a constant magnitude of ampere turns, the force exerted on the armature by the electromagnet will increase exponentially as the length of air gap decreases.

Cantilever mounted leaf spring 35 coacts with fixed stop 44 to apply a force to armature 28 that opposes the force of tension spring 47. The forward, free end of leaf spring 35 engages stop 44 and is deflected thereby a maximum amount whenever armature 28 is in it reset position, as can be seen in Fig. 2. As the armature is pivoting toward its pickedaup position, the fixed end of spring 35 moves in a manner to decrease the deflection. When the armature reaches a position immediately preceding the engagement of the movable and fixed contacts, 39 and 43 respectively, guard 36 engages spring 35, and further movement of the armature will cause guard 36 to carry the spring 35 out of engagement with stop 44 thereby inactivating the force of this spring. The amount of force exerted by spring 35 will vary in a linear relation to its deflection, therefore its greatest force will be exerted when the armature is reset. This force aids the force of the electromagnet and enables the electromagnet to pick up the armature with a smaller quantity of flux than would otherwise be possible. As the armature moves away from its reset position, the electromagnetic force increases exponentially thereby offsetting the linear decrease of the aiding force of spring 35 and counterbalancing the linear increase of the opposing bias force of spring 47.

As armature 28 approaches it picked-up position, movable switch contacts 39 will engage cooperating fixed switch contacts 43. As the armature 28 continues moving, until plate 30 engages the front edge of pole head 16, cantilever mounted resilient switch member 34 will yield and the cooperating switch contacts will wipe thereby establishing contact pressure which reacts to exert a force on the armature tending to pivot the armature toward its reset position. The picked-up position of armature 28 is illustrated in Fig. 3.

To pick up the armature 28 from its reset position, the electromagnet must produce a value of air gap flux sufficient to create with the aid of leaf spring 35 a force which will overcome the bias force of spring 47 and the force of gravity. The magnitude of ampere turns in winding 12 necessary to produce this flux can be varied by adjusting core element 18 to vary the reluctance of core member 14. However, the position of core element 18, which influences the pick-up or attractive effect of the electromagnet, will be fixed and unchangeable during the operation of the electromagnetic relay.

The armature 28 will pick up whenever the current from the remote source of alternating current increases to a value required to produce the given magnitude of ampere turns necessary for pick-up. After the armature 28 has picked-up, because of the exponential increase of magnetic force, this given magnitude of ampere turns will result in a force exceeding that required to hold the armature against the opposing combination of bias, gravity and contact forces. The holding force exerted on the picked-up armature by the given magnitude of ampere turns is determined by the flux in the air gap between the coplanar pole faces 22 and 23 and the inclined plate 30. This flux, and thus the holding effect of the electromagnet, can be varied by adjusting the radial position of pole head 16.

With armature 28 picked-up, the air gap has a tapered or wedge configuration, as can be seen in Fig. 3. There is no air gap at the forward edge of pole head 16, which engages armature 28, while there is a gap of maximum length at the rear edge of the pole head. Rotation of the pole member 14 will vary the relative length of air gap in the path of each line of flux issuing from both pole faces. The greater the average length of air gap under the shaded pole face 23, the greater the reluctance to the out-of-phase component of flux and the less will be the relative magnitude and the smaller will be the phase angle of this component. The smaller the phase angle and magnitude of the out-of-phase component of flux, the greater will be the unbalance between the two components of flux and the greater will be the tendency of the armature to drop out during the low point in each cycle of magnetic force.

It is possible, therefore, to change the holding effect of a particular quantity of ampere turns by rotating the pole member 14. It has been found that the holding effect can be adjusted to release the armature for resetting at a quantity of ampere turns which is only a small amount less than the given magnitude necessary for pick-up, While quiet operation of the relay is maintained without objectionable chattering. To obtain this desired result, the pole head 16 can be adjusted to any position between the two positions illustrated in Fig. 4 by the solid and broken-lined shading coil 25. Rotating the pole member 14 until the maximum air gap is under the shaded pole face 23 will increase further the reset quantity of ampere turns but at the same time may increase chattering to an excessive degree.

While I have shown and described a preferred form of my invention by way of illustration, many modifications will occur to those skilled in the art. I therefore contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electromagnetic device comprising an alternating current electromagnet having an adjustable core member of predetermined diameter and including means to provide magnetic fields of different phase, and a movable armature wider than said predetermined diameter pivotally connected to said electromagnet and held by the attraction of said magnetic fields in a position to define with said core member an air gap of tapering length, said gap providing air paths of different average lengths for said magnetic fields, said core member being adjustable with respect to said armature to change the average length of air path of one of said magnetic fields with respect to the average length of air path of the other magnetic fields.

2. An electromagnetic device comprising an energizing winding supplied with alternating current, a core member for said winding having a shading coil to establish magnetic fields of different phase, a magnetizable frame member rotatably supporting said core member, and a movable magnetizable armature pivotally connected to said frame member and held by the attraction of the magnetic fields in a position defining an oblique angle with the axis of said core, said core member being rotatable about its axis to obtain recipiocal adjustment in the relative lengths of the air paths followed by said magnetic fields between said core member and said armature of relative variation of the reluctance to said magnetic fields thereby to vary the holding effect of said magnetic fields upon said armature.

3. An electromagnetic device comprising a magnetizable frame member, a magnetizable core having one end rotatably mounted on said frame member, a plurality of projecting coplanar pole faces formed at the other end of said core, a shading coil encircling at least one of said pole faces, an energizing winding encircling said core and supplied with alternating current, a movable magnetizable armature pivotally connected to said frame member in cooperating relationship with said pole faces and movable toward said pole faces to an attracted position in response to energization of said winding, said armature in its attracted position engaging only the perimeter of said other end of said core to define an oblique angle with the plane of said pole faces whereby rotation of said core will vary the relative average length of air gap between each pole face and said armature, and means biasing said armature away from said pole faces.

4. An electromagnetic device comprising an energizing winding, a core for said winding, a magnetizable frame member supporting said core, a magnetizable armature disposed in cooperating relation with said core and frame member and movable between an attracted and an unattracted position, means biasing "said armature to said unattracted position, a fixedly located stop member engageable with said armature to define said unattracted position, and a spring means mounted on said armature member fixedly mounted on said frame member and engageable with said armature to define said unattracted position, a resilient cantilever spring member mounted on said armature and deflected by said stop member to establish a force opposing the force of said biasing means whenever said armature is in said unattracted position, and a cooperating rigid member mounted on said armature and disposed to carry said spring member in a partially deflected state away from said stop member as the armature approaches its attracted position thereby inactivating said opposing force.

6. An electromagnetic device comprising an energizing winding, 21 core member for said Winding having a shading coil to establish magnetic fields of different phase, a magnetizable frame member rotatably supporting said core member, a movable magnetizable armature pivotally connected to said frame member and held by the attraction of the magnetic fields in an attracted position to define with said core member an air gap of tapering lengths, said core member being rotatable about its axis to permit adjustment in the holding effect of said magnetic fields upon said armature, means biasing said armature to an unattracted position, a stop member fixedly mounted on said frame member to define said unattracted position, and a spring means mounted on said armature and releasably engageable with said stop member to supply a force opposing the force of said biasing means.

7. An electromagnetic device comprising an energizing winding supplied with alternating current, a core member for said winding having a shading coil thereby to establish two magnetic fields of different phase, a magnetizable frame member rotatably supporting said core member, a magnetizable armature pivotally connected to said frame member and movable between an attracted and an unattracted position, said armature held by the attraction of said magnetic fields in said attracted position wherein said armature defines an oblique angle with the axis of said core thereby to form air paths of tapering lengths for said magnetic fields, said core member being rotatable about its axis to permit adjustment in the average length of air path of one of said magnetic fields with respect to the average length of air path of the other magnetic field thereby to vary the holding effect of said magnetic fields upon said armature, means biasing said armature to said unattracted position, a stop member fixedly mounted on said frame member to define said unattracted position, and a spring means mounted on said armature and releasably engageable with said stop member to supply a force opposing the force of said biasing means.

8. An electromagnetic relay comprising a U-shaped magnetizable frame member having first and second legs, a source of alternating current, an energizing winding connected to said source, a core member for said winding mounted at one end on said frame member for rotary movement about its axis and disposed substantially parallel to and spaced between said first and second legs, a pair of coplanar pole faces projecting from the other end of said core member to provide two independent flux paths, a shading coil encircling one of said pole faces to delay the flux following the path through said shading coil, a movable armature member pivotally connected to said frame member at the open end of said first leg and disposed in a generally transverse direction across said pole faces to move toward said pole faces to an attracted position in response to magnetic attraction produced by energization of said winding, said armature member including a magnetizable plate cooperating with said pole faces to provide an air gap of tapering length in said independent paths of flux whenever said armature is in its attracted position, whereby rotary movement of said core member will vary the relative average length of air gap in each independent flux path thereby varying the reluctance to the flux following each path, an insulating member mounted on said plate and carrying a resilient switch member having one end fixed to said insulating member and an opposite end supporting at least one movable switch contact, an insulating bracket mounted on said second leg and supporting a fixed stop and at least one fixed switch contact in spaced apart cooperating relation on opposite sides of said movable switch contact, said movable and fixed switch contacts releasably engageable whenever said armature member is in its attracted position, a first spring means providing a bias force to urge said armature member away from said pole faces to a reset position defined by said movable switch contact releasably engaging said stop, and a second spring means mounted on said insulating member and releasably engageable with said stop to provide a force opposing said bias force.

9. An electromagnetic switch comprising an energizing Winding, a core for said Winding, a magnetizable frame member supporting said core, a magnetizable armature disposed in cooperating relation with said core and frame member and movable between an attracted and an unattracted position, means biasing said armature to said unattracted position, a relatively stationary switch contact mounted on said frame member, said armature including a cooperating switch contact disposed for engagement with said relatively stationary switch contact when ever said armature is in its attracted position, a fixedly located stop member engagable with said armature to refine said unattracted position, and spring means releasably interconnecting said armature and a non-com ductive part of said stop member whenever said armature is in its unattracted position to supply a force opposing the force of said biasing means.

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